1. Field of the Invention
The present invention relates to systems using electromagnetic transponders, that is, transceivers (generally mobile) capable of being interrogated in a contactless and wireless manner by a unit (generally fixed), called a read/write terminal. The present invention more specifically relates to transponders having no independent power supply. Such transponders extract the power supply required by the electronic circuits included therein from the high frequency field radiated by an antenna of the read/write terminal. The present invention applies to such transponders, be they read-only transponders, that is, transponders for operating with a terminal that only reads the transponder data, or read/write transponders, which contain data that can be modified by the terminal.
The present invention more specifically relates to the detection by a transponder of the distance separating it from a terminal and, more specifically, of the transponder position with respect to a distance threshold of the terminal conditioning the system operation.
2. Discussion of the Related Art
Electromagnetic transponders are based on the use of oscillating circuits including a winding forming an antenna, on the transponder side and on the read/write unit side. These circuits are intended to be coupled by a close magnetic field when the transponder enters the field of the read/write unit. The range of a transponder system, that is, the maximum distance from the terminal at which a transponder is activated (awake) depends, especially, on the size of the transponder antenna, on the excitation frequency of the coil of the oscillating circuit generating the magnetic field, on the intensity of this excitation, and on the transponder power consumption.
FIG. 1 very schematically shows, in a functional way, a conventional example of a system of data exchange between a read/write unit 1 (STA) and a transponder 10 (CAR).
Generally, terminal 1 is essentially formed of an oscillating circuit formed of an inductance L1 in series with a capacitor C1 and a resistor R1, between an output terminal 2p of an amplifier or antenna coupler 3 (DRIV) and a terminal 2m at a reference potential (generally, the ground). Amplifier 3 receives a high-frequency transmission signal Tx, provided by a modulator 4 (MOD). The modulator receives a reference frequency, for example, from a quartz oscillator 5 and, if necessary, a data signal to be transmitted. In the absence of a data transmission from terminal 1 to transponder 10, signal Tx is used only as a power source to activate the transponder if said transponder enters the field. The data to be transmitted generally come from a digital electronic system, for example, a microprocessor 6 (xcexcP).
The connection node of capacitor C1 and inductance L1 forms, in the example shown in FIG. 1, a terminal for sampling a data signal Rx, received from a transponder 10 and intended for a demodulator 7 (DEM). An output of the demodulator communicates (if necessary via a decoder (DEC) 8) the data received from transponder 10 to microprocessor 6 of read/write terminal 1. Demodulator 7 receives, generally from oscillator 5, a clock or reference signal for a phase demodulation. The demodulation may be performed from a signal sampled between capacitor C1 and resistor R1 and not across inductance L1. Microprocessor 6 communicates (bus EXT) with different input/output (keyboard, screen, means of transmission to a provider, etc.) and/or processing circuits. The circuits of the read/write terminal draw the power necessary for their operation from a supply circuit 9 (ALIM), connected, for example, to the electric supply system.
On the side of transponder 10, an inductance L2, in parallel with a capacitor C2, forms a parallel oscillating circuit (called a reception resonant circuit) intended for capturing the magnetic field generated by series oscillating circuit LIC1 of terminal 1. The resonant circuit (L2, C2) of transponder 10 is tuned on the frequency of the oscillating circuit (L1, C1) of terminal 1.
Terminals 11, 12, of resonant circuit L2C2, which correspond to the terminals of capacitor C2, are connected to two A.C. input terminals of a rectifying bridge 13 formed, for example, of four diodes D1, D2, D3, D4. In the representation of FIG. 1, the anode of diode D1 and the cathode of diode D3 are connected to terminal 11. The anode of diode D2 and the cathode of diode D4 are connected to terminal 12. The cathodes of diodes D1 and D2 form a positive rectified output terminal 14. The anodes of diodes D3 and D4 form a reference terminal 15 of the rectified voltage. A capacitor Ca is connected to rectified output terminals 14, 15 of bridge 13 to store power and smooth the rectified voltage provided by the bridge. It should be noted that the diode bridge may be replaced with a single-halfwave rectifying assembly.
When transponder 10 is in the field of terminal 1, a high frequency voltage is generated across resonant circuit L2C2. This voltage, rectified by bridge 13 and smoothed by capacitor Ca, provides a supply voltage to electronic circuits of the transponder via a voltage regulator 16 (REG). These circuits generally include, essentially, a microprocessor (xcexcP) 17 (associated with a memory not shown), a demodulator 18 (DEM) of the signals that may be received from terminal 1, and a modulator 19 (MOD) for transmitting information to terminal 1. The transponder is generally synchronized by means of a clock (CLK) extracted, by a block 20, from the high-frequency signal recovered across capacitor C2 before rectification. Most often, all the electronic circuits of transponder 10 are integrated in a same chip.
To transmit the data from transponder 10 to unit 1, modulator 19 controls a stage of modulation (back modulation) of resonant circuit L2C2. This modulation stage is generally formed of an electronic switch (for example, a transistor T) and of a resistor R, in series between terminals 14 and 15. Transistor T is controlled at a so-called sub-carrier frequency (for example, 847.5 kHz), much smaller (generally with a ratio of at least 10) than the frequency of the excitation signal of the oscillating circuit of terminal 1 (for example, 13.56 MHz). When switch T is closed, the oscillating circuit of the transponder is submitted to an additional damping as compared to the load formed of circuits 16, 17, 18, 19 and 20, so that the transponder draws a greater amount of power from the high frequency field. On the side of terminal 1, amplifier 3 maintains the amplitude of the high-frequency excitation signal constant. Accordingly, the power variation of the transponder translates as an amplitude and phase variation of the current in antenna L1. This variation is detected by demodulator 7 of terminal 1, which is either a phase demodulator or an amplitude demodulator. For example, in the case of a phase demodulation, the demodulator detects, in the sub-carrier half-periods where switch T of the transponder is closed, a slight phase shift (a few degrees, or even less than one degree) of the carrier of signal Rx with respect to the reference signal. The output of demodulator 7 (generally the output of a band-pass filter centered on the sub-carrier frequency) then provides an image signal of the control signal of switch T that can be decoded (by decoder 8 or directly by microprocessor 6) to restore the binary data.
It should be noted that the terminal does not transmit data when it receives some from a transponder, the data transmission occurring alternately in one direction, then in the other one (half-duplex).
FIG. 2 illustrates a conventional example of data transmission from terminal 1 to a transponder 10. This drawing shows an example of shape of the excitation signal of antenna L1 for a transmission of a code 1011. The modulation currently used is an amplitude modulation with a 106 kbit/s rate (a bit is transmitted in approximately 9.5 xcexcs) much smaller than the frequency (for example, 13.56 MHz) of the carrier coming from oscillator 5 (period of approximately 74 ns). The amplitude modulation is performed either in all or nothing or with a modulation ratio (defined as being the difference of the peak amplitudes between the two states (0 and 1), divided by the sum of these amplitudes) smaller than one due to the need for supply of transponder 10. In the example of FIG. 2, the carrier at 13.56 MHz is modulated in amplitude, with a 106-kbit/s rate, with a modulation rate tm of 10%.
FIG. 3 illustrates a conventional example of a data transmission from transponder to terminal 1. This drawing illustrates an example of the shape of the control signal of transistor T, provided by modulator 19, for a transmission of a code 1011. On the transponder side, the back modulation is generally of resistive type with a carrier (called a sub-carrier) of, for example, 847.5 kHz (period of approximately 1.18 xcexcs). The backmodulation is, for example, based on a BPSK-type (binary phase-shift keying) coding at a rate on the order of 106 kbits/s, much smaller than the sub-carrier frequency.
It should be noted that, whatever the type of modulation or back modulation used (for example, amplitude, phase, frequency) and whatever the type of data coding (NRZ, NRZI, Manchester, ASK, BPSK, etc.), this modulation or back modulation is performed digitally, by jumping between two binary levels.
The oscillating circuits of the terminal and the transponder are generally tuned on the carrier frequency, that is, their resonance frequency is set on the 13.56-MHz frequency. This tuning aims at maximizing the energy diffusion to the transponder, generally, a card of credit card size integrating the different transponder components.
In some applications, it may be desired to know the distance separating the transponder from a terminal, or the position of the transponder with respect to a distance threshold. Such a distance detection can be used, for example, to switch the system to an operating mode or another according to whether the transponder is close (on the order of 2 to 10 cm) or very close (less than approximately 2 cm) to the reader. The notion of proximity involves the distance separating antennas L1 and L2 from each other.
Document WO-A-97/34250 provides a device of contactless information exchange with an electronic label, this device including means for preprocessing a signal representative of the distance between the label and the device, based on the signal transmitted by the label. These means are used to determine and to signal, to the information exchange device, that the information coming from the label is included in a window of predetermined value. The device described by this document uses a measurement of the amplitude of a low-frequency modulation provided by the label responsive to a read control signal sent by the device. According to this document, the amplitude of this modulation is representative of the distance separating the label from the information exchange device.
In addition to the fact that the distance detection of this document is performed on the terminal side, this detection requires a demodulation of the back-modulated signal transmitted by the transponder as well as a preprocessing of the demodulated signal to extract the distance information therefrom.
The present invention aims at providing a novel solution of distance measurement between a transponder and a reader. In particular, the present invention aims at providing a solution that is implemented on the transponder side and not on the read/write terminal side.
The present invention also aims at providing a solution that enables easily detecting the position of a transponder with respect to a distance threshold separating it from the terminal.
The present invention also aims at a distance threshold that is self-adaptive according to the system environment.
To achieve these and other objects, the present invention provides a method of determining the distance separating an electromagnetic transponder from a terminal generating a magnetic field by means of a first oscillating circuit, the transponder including a second oscillating circuit, upstream of a rectifying means adapted to providing a D.C. voltage, the method comprising the steps of:
storing a first information relative to the level of the D.C. voltage when the second oscillating circuit is tuned on a determined frequency;
storing a second information relative to the level of the D.C. voltage after having caused a frequency detuning of the second oscillating circuit; and
comparing the two stored pieces of information.
According to an embodiment of the present invention, the measurements are periodically performed and the variation of the stored information is compared for two successive measurements in the same tuning conditions.
According to an embodiment of the present invention, said pieces of information represent the respective values of the D.C. voltage.
According to an embodiment of the present invention, the comparison between the first and second pieces of information is used to determine the position of the transponder with respect to a critical coupling position of the respective oscillating circuits of the transponder and of the terminal.
According to an embodiment of the present invention, the determined frequency corresponds to the excitation frequency of the oscillating circuit of the terminal for the remote supply of the transponder.
According to an embodiment of the present invention, the method is applied to determining the operating mode of the transponder among two modes respectively corresponding to a tight coupling or loose coupling position.
The present invention also relates to an electromagnetic transponder including at least one switched capacitor to implement the method of the present invention.
According to an embodiment of the present invention, the transponder includes a capacitor, in parallel with an inductive element of the second oscillating circuit, and in series with a switching means, the rectifying means being formed of a one-way conduction element.
According to an embodiment of the present invention, the transponder includes two capacitors, respectively associated with each end terminal of an inductive element of the second oscillating circuit, each capacitor being connected in series with a switching means, a reference terminal of which is connected to a reference supply potential of the electronic circuit, downstream of the rectifying means.
According to an embodiment of the present invention, the transponder further includes two resistive modulation means, in parallel with a capacitor for smoothing the rectified voltage provided by the rectifying means.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.